This invention pertains to fixed offshore platform construction and erection and, more specifically, to a simplified, cost effective method of erecting an offshore platform that requires the driving of only two piles in a manner which allows all welds to be made above the water line.
Offshore platforms can be categorized generally as being submersible, semisubmersible, jackup or fixed. Although all have their advantages and advocates, fixed platform construction is in widespread use.
Minimal structure fixed platforms are advantageous in shallow water, calm-weather regions such as the Gulf of Mexico where these platforms are installed in depths of 30-300 feet. Because these minimal structure platforms cost much less than a traditional full structure platform, they are particularly favored by cost-conscious and tight-budget operators.
There are numerous varieties of minimal structure platforms offering different load capabilities at a range of depths. Almost all consist of a jacket including tubular sleeves for receiving piles driven through the sleeves and into the seabed floor. Once the pilings have been driven to grade in the seabed, most designs fix the jacket to the pilings by some combination of grouting, bolting, or welding. The upper decking and equipment are then mounted to the jacket to complete the platform.
Minimal structure platforms differ from full structure platforms because minimal structure platforms are constructed with a smaller jacket assembly and fewer pilings. Some use the drive pipe, or caisson, as one of the legs for support. For instance, on information and belief, Barnett and Casbarian, Inc. (Metarie, La.) has designed a platform that is clamped to the side of the caisson at points above and below the water line. Piles are then driven through three sleeves to anchor the platform adjacent the caisson. CBS Engineering (Houston, Tex.) markets a design of minimal structure platform known as the Moss III, which uses a tripod support system with the caisson serving as one of the support legs.
Other minimal structure platforms include a so-called chopped tripod that is designed, on information and belief, by Mustang Engineering (Houston, Tex.) and which uses a tripod base to support the caisson. By supporting the caisson in that manner, the caisson can be used as the sole support for a platform. Likewise, the Moss II (CBS Engineering (Houston, Tex.)) and so-called T-Horse (Atlanta Engineering (Houston, Tex.)) are designs which rely on the caisson as the sole support.
Although all of these minimal structure platforms are easier and cheaper to construct and erect than a full structure platform, the method of the present invention offers several significant advantages over these other minimal structure platforms. For instance, it is a feature of the present invention that all required connections can be made by welding and that all welds are made above water. The present invention does not require any grouting, bolting, clamping, or underwater welding. Because all of the connections are above water welds, the method of the present invention simplifies the erection of the platform.
Minimal structure offshore platforms of the type which allow all welds to be made above the water line have been designed by Petro-Marine Engineering of Texas, Inc. (Houston, Tex.) and several, know as "Guardian" platforms, have been erected in the Gulf of Mexico. In erecting those platforms, which is accomplished in part by driving a pile section through an angled and canted sleeve and then welding another pile section onto the section driven through the sleeve and repeating those steps until the pile is driven to grade, a problem was encountered with alignment of successive pile sections and the effect of that alignment problem on the stance of the platform. Specifically, because the force of gravity pulls a pile section straight downwardly while the sleeve in which it is positioned is angled and canted with respect to the vertical, the pile section tends toward a slight angle with respect to the axis of the cylinder represented by the sleeve. That tendency makes alignment of a pile section driven through the sleeve with the next pile section to be welded in place more difficult; precise alignment is almost impossible. Consequently, as successive pile sections are welded in place, the pile begins to curve away from the axis of the sleeve with the result that the triangular stance of the platform on the ocean floor is of smaller dimensions, decreasing the stability of the platform. Further, even if precise alignment could be effected, the result would still be a pile which droops, e.g., tends downwardly away from the axis of the sleeve simply because of the effect of gravity.
It is, therefore, another feature of the method of the present invention to align the pile sections by using a shim mounted to the high side of each of the pile sleeves. These shims also act as a guide for the piles during driving to ensure that the piles enter the seabed floor at the proper angle.
It is still another feature of the present invention to insert shims between the piles and the pile sleeves to aid in centering the piles in the sleeves. Centering the piles in the pile sleeves is advantageous in making the weld between the pile and the pile sleeve and makes the weld stronger.
It is still another feature of the present invention to mount a plurality of shims to the top of the caisson sleeve. The shims maintain proper spacing between the caisson and the template including the pile sleeve so as to facilitate the welding of the caisson to the template.